Pelletization of phosphate shale



Jan. 8, 1957 c. MARCELLUS ET AL 2,776,828

PELLETIZATION OF PHOSPHATE SHALE Filed Fb. 27, 1953 M 1 (WW;

. anawe us By foodward Jam/ir/c Uit PELLETIZATION F PHOSPHATE SHALE Application February 27, 1953, Serial No. 339,354

4 Claims. (Cl. 263-53) This invention relates to the pelletization of phosphate shale to produce feed for electric phosphate smelting furnaces for the production of elemental phosphorus or phopshate compounds.

Electric phosphate smelting furnaces require a feed of phosphatic ore which has been calcined to decompose hydrated and carbonaceous material and which is of relatively coarse particle size, in the range of plus 6 mesh to 3 inches, to allow the escape of gases, generated or introduced during the smelting operation, through the furnace burden with a minimum of resistance. When the proportion of small material in the charge is too large, resistance to gas flow through the charge is high and there is a tendency of the fine material to fuse and arch above the slag pool. Gas pressure then increases until it is suflicient to break through the obstruction and thus causes puffs or blows in the furnace. Material passing a 3 inch screen and retained on a 6 mesh screen is satisfactory for use in an electric phosphate smelting furnace, but the proportion of smaller size material must not be large.

Considerable difiiculty has been encountered in the economical production of suitable calcined feed for electric phosphate smelting furnaces from phosphate shales such as occur in the Western portion of the United States, as tyified by those found in the vicinity of Pocatello, Idaho. The shale-like character of the phosphate deposits found in the western part of the United States and particularly in the States of Utah, Wyoming, Idaho and Montana is distinctly different than the sand-like phosphate deposits found in the southeastern part of the United States.

The shale as it is received from the mines, after crushing to minus 3 inch size, normally contains on the order of 60% minus inch particles and on the order of 40% of compacted rocks of a size of 4 inch to 2 inches or more. If this material is fed to a calcining kiln operated at a hot zone temperature sufficient to effect heat nodulization of the shale, the amount of fine particle size material in the shale is so large that it cannot all be formed into satisfactory nodules or pellets by building on the coarse particles, and the result is that the dust forms a layer on the walls of the kiln which rapidly reaches incipient fusion, further lump shale becomes embedded in the fused layer or sticky mass and an adherent scale builds up rapidly in the hot zone of the kiln which requires reaming, shooting or boring of the kiln every two or three hours in order to prevent eventual plugging of the kiln.

This reaming or boring operation interferes with regular production, produces non-uniform product discharge, and also results in the discharge from the kiln of large slugs of fused calcined scale which must be cooled, broken and re-screened before feeding to the electric furnaces.

Alternatively, when the entire mine run product, crushed and ground to pass through a inch or inch mesh screen, is fed to the calcining kilns and then calcined at a temperature which produces sintered nodules of the size required for electric furnace smelting, the fine O ice material sifts through the coarser pieces to the walls of the kiln, and the lower melting constituents of the fine and coarse fractions are heated to the fusionpoint. This causes stickiness and adherence of both fine and coarse particles to the Walls of the kiln, so that a scale or ring builds up rapidly on the walls of the hot zone of the kilns, requiring frequent rodding, boring or shooting for the removal of thescale to keep the kiln open.

The delay occasioned by the removal of the scale or rings interferes with the regularity of the kiln operation and leads to non-uniform kiln discharge products. The large nodules of fused material, which may range from one foot to several feet in diameter, discharged as a result of the rod'ding or boring operations, are difficult to cool and break to suitable size for furnace feed, and the handling of these large nodules of hot material 'is extremely hard on the kiln andconveying equipment. Large balls are also formed by agglomeration of the fused material in the bed and these too are difficult to handle and put undue strain on the kilns and handling equipment.

When the whole phosphate shale is crushed to minus inch, moistened, passed through a pug mill and formed into pellets of /2 inch to 1 /2 inch size and the pellets dried and calcined in calcining kilns, much of it does not contain sufficient clayey material to act as a binder and the pellets are hence structurally too weak to Withstand mechanical handling and conveying, kiln calcination and the necessary handling and compression pressures encountered in feeding them to the furnaces.

Washing the western phosphate shales, as is the practice in producing phosphorus and phosphate products from the phosphate sands in the Tennessee Valley phosphate deposits, as described for example in Chemical Engineering Report No. 4 of the Tennessee Valley Authority published in 1950, is impractical and does not produce a product which can be converted intoa suitable furnace feed.

It is, therefore, an object of this invention to provide a method and process by which the entire phosphate ore used for electric smelting furnace feed, including both the coarse and fine fractions, from western phosphate shales may be calcined to produce suitable electric furnace feed and phosphorus orphosphate products.

Another object of this invention is to provide a method of calcining phosphate shales of the type found in the State of Idaho and other western states which will provide satisfactory and economical operation of the calcining kiln without the necessity for frequent boring and reaming of the kilns and without the production of excessively large lumps of calcined shale which need to be broken after removal from the calcining kiln and before feeding to the electric furnace.

Another object of this invention is to provide a method of concentrating the clayey or binder content of Idaho phosphate shales in the fine particle size fraction of these shales so as to provide sufficient binder to bind the fine particle size fraction into pellets of sufficient strength after calcining to withstand handling and pressures to which they are subjected in feeding to the electric furnaces.

Another object of this invention is to provide a method of preparing and calcining western phosphate shales for electric furnace feed whichwill permit operation of the calcining kilns at lower temperatures than possible in the heat nodulization of the whole shale and at the same time provide a more satisfactory product and better kiln operating conditions.

Various other objects and advantages of this invention will appear as this description proceeds.

We have discovered a method whereby the whole of the raw phosphate as received from the western phosphate deposits, such as found for examplein southern Idaho, may be economicallyprocessed to produce a suitable feed of sufficiently uniform size for electric phosphate smelting furnaces.

Asindicated .-above, efforts vtopelletize the crushed. and ground entire :mine runproducL-gave results ranging: from largely unsatisfaotorynp to totally unsuccessful, depending .upontheparticularsourceand character of the ore. Likewise,;efforts to crush;and grind only-the coarse fraction ofthemine output and then pelletize this material were also unsuccessful.

Surprisingly enough, however when the fines fraction from the .screening of :theucrushed mine run shale was pelletized it was foundthatthis material pelletized satisfactorily and.formedcalcinethpellets.of sufiicient strength to withstand subsequent furnacing operations.

,This discovery has made possible the :practical utilization of the-largeproportion of fines encountered in. the typical: processing of 1W6Sl161'1] shales, .and has eliminated to a :great-extent the ;difiiculties described above which were associated withx prior :artsefiorts to process the mine run product.

The utilization of our discovery has even made tunnecessary the .use oftadditional'hinder material 'or defiocculating agents, rexpedients which have occasionally beenusedxin an effort to overcome the difficulties of the prior art However, deflocculantssuchas sodium carbonate, sodium hydroxide and sodium polyphosphates, or the like, may be added for betterdefiocculation and'dispersion of the clayey or binder content of the mineral, and additional binders may,- of course, be used if desired.

In the practiceof our invention, ..we, therefore, take the whole ofthe western phosphate shale as it comes from the mine and, after crushingto minus 3 inch size, separate it'into two fractions consisting ofapproximately 40% of a coarse fraction which'may range from a minimum of A to /2 inch to a maximum of 2 to 3 inch size particles, and approximately 60% of a fine particle size fraction under minus' flvto minus /2 inch size particles. The large size particles are fed to calcining kilns 'for calcination at suitable temperature conditions to decompose the hydrated and carbonaceous material and to sinter the material into pellets of sizes satisfactory for electric furnace feed. The fine particle size fraction is mixed with sufiicient water (of the order of 12 to 15%) to produce satisfactory wet pellets of .the order of /1 inch to 1 /2 inches in diameter, pelletized and dried in a quiescent bed'to a moisture content of lessithan 6% and preferably less than 4%, and .these pellets calcinedto decompose the hydrated and carbonaceous material thereinandform a satisfactory electric furnace feed. The .pelletized fine fraction contains sufiicient clayey'binder (measured as .particles of minus 200 mesh size) to form pellets of sufficient strength to withstand normal calcining, handling and furnace operations and produce calcined pellets of sizes suitable 'for feeding .to electric furnaces, and the removal of'the fines and dust from the.coarse fractionsproduces a product whichcan be uniformly calcined tollikewiseproduce a satisfactory feed for the electric smelting furnaces without excessive scale or ring formation on the walls of the kilns.

In the passage of both the coarse fraction and the pelletized fraction through the calcining kilns, vcertain breakage and dusting inevitably occurs. The amount of this breaking and dusting is relatively small .andithe broken and clustedparticles willagglomerate:andadhere to thelargerparticles tothe vvextentthat the build-up .of a fused scaleon .the-walls. .of.the.kiln.in thehot zone of the kilns is very materially reduced.

Whereas; .wh en operating on whole shale, 1 build-up was so great as to require reaming=or-boring:of .the hotzone of the, kiln-every two or .three;hours,. whenaoperatingiwith separate 1calcining.ofc.the coarse particlesize fraction and theapelletized :finetparticle size fraction, it is :not necessary to ream or bore the hot zone of the kiln more than once a day and no shooting of the scale or ring formation is necessary. This reduced scale build-up permits steadier operation of the kilns and reduces the formation of large fused nodules, which are dislodged in the boring operation, and, therefore, reduces the strain on the kilns and conveying equipmentcansed by these large fused nodules. The steady operation of the kilns results in higher pro duction of. calcined pebbles per operating hour and higher operating time giving a greatly-increased capacity for the same equipment.

In addition to more satisfactory operation of the calcining kilns, it is possible to operate the calcining kilns at 150 to 300 C. lower bedtemperature than when operating on whole raw shale. In order to produce satisfactoryfurnace feed charge when operating on whole raw shale, it was necessary to operate the calcining kilni-at abed orzhot zone temperature of approximately 1300 to 1350 C., which also increased the tendency toward incipient fusion of the dust layer and lower melting constituents and scale or ring formation, whereas when operating on. coarse and fine fractions of raw shale in-whichwtheafinefraction has been'separated and wet pelletized, satisfactory calcining conditions may be obtained with ta'bed or hot zone temperature of approximately 1050 =to '1200 C. Thisreduced temperature not only provides fuel economy, but permits a greater amount of shale to-beheated to the required temperature andpassed .throughthe'kilns in a, given period and, therefore, increases .the kiln capacity.

"Wherewthe wholeeshalecontains of the order of 18% of;claybearingt or rninus 200 mesh material, the minus inch, fraction :will contain about 30% of claybearing or. minus 200-mesh. material and the coarse fractionwill containlabout18% ofaclaybearing or minus 200 mesh material. while the minus200 mesh material is not a direct measure of the availableclay binder in the shale, it is proportionalithereto and provides a convenient measurement 'of theflbindingqproperties of the fine particle size fraction.

. :Bytthusconcentrating the clayey or minus 200 mesh material. inltherfine fraction sufiicient binder is .provided inxthisfraction to satisfactorily bind the entire fine or minus A :to minus /2 inch fraction so that it can be formed into 'wetpellets of suitable size for the electric furnace z-feed, vandgwhich, when dried and calcined, will be of; suflicientstrength to withstand handling and furnacing operations.

Where-the whole shalecontains more'than-l8% of clayey (minus :ZOOImesh) material, when separated into two fractions containing plus inch or plus /2 inch material and .minus or minus /2 inch material, the fine .fraction, :which normally runs about 60% of .the whole crushedtzshale, "Will have a larger percentage of minus :200umesh or.clayey material and the coarse fraction -will-have 'a-smaller percentage .of :clayey material or hinder. iIn-either case,.by. separating the .finefraction from the coarsefraction of the shale, sutficient binder material will berconcentrated in the fine fraction topermit satisfactoryproduction of pellets by wetpelletization and calcining, and'the coarse fraction is of suflicientstrength andsufiiciently freeof dust to'permitcalcination without unduescaling-of thecalcining kilns or other complications.

Referring now to {the drawings which illustrate one methodof operating our process, the figureis a schematic layout of; aplant' design embodyingthe use of our process.

.In" the layout indicated, l the r-aw shale -as. it comes from r the; minesorwstoekpiles passes over avibrating or shaking 5 by any suitable feeding means. The fine particle size fraction consisting of minus inch or minus /2 inch material, including the dust from the coarser particles passing through the screen 1, is deposited on a belt or other conveyor 6 and passes into a pug mill or other mixing device 7 where water from the line 7a equal to about 12% to 15% of the fine particle shale is added and the material mixed. From the pug mill the particles pass into a rotating inclined pelletizer 8 which has suitable inclination and is rotated at a suitable speed to roll the particles of mixed shale into balls or pellets preferably from /2 to 1 inches in diameter. The pelletizer 8 may be heated if desired to prevent the moist pellets from sticking to it and to give a dry skin coating to the pellets.

At the lower end of the pelletizer 8, a screen or perforations 9 permits material of minus /2 inch diameter to drop through the screen. The perforations or screen 10 at the lower end of the pelletizer 8 permits pellets of /2 to 1 /2 inches in diameter to pass through and the oversized material is discharged at the end of the pelletizer. The over-sized and under-sized material may be conveyed by any suitable means, such as belt conveyors 16, 17, 18 and 119, back to the belt 6 and passed again through the pug mill 7. The pellets of /2 inch to 1 /2 inches in diameter drop onto the belt 11 and are conveyed through a tunnel or other form of quiescent drying kiln 12 in which the moisture is reduced to below 6% and preferably to below 4%. From the end of the kiln 12 the pellets are elevated by means of a bucket elevator or the like 13 and are deposited in a bin 14 from which they may be conveyed into a rotary kiln 15. Tray, chain grate or pallet dryers are preferably used to dry the pellets in quiescent, thin layers until they have become hardened sulficient to withstand the subsequent handling which they must undergo in the calcining and furnacing operations.

In passage through the kilns 5 and 15, the phosphate shale is heated to a temperature preferably of above 105 C. which is sufficient to decompose the hydrated and carbonaceous material therein and produce sintered nodules suitable for electric furnace smelting. As the dust has been removed from the coarse particle size fraction and the fine particle size fraction, including the dust, has been moistened, mixed and pelletized, there is much less dusting of the shale particles in their passage through the kilns and much less build-up of scale on the walls of the kilns in the hot zones thereof than in the calcining of the whole raw shale. The absence of a fused layer adjacent the walls of the kiln permits more ready transfer of heat from the flame to the tumbling bed of lump material and the fact that the lumps of shale in the absence of a fused scale layer can be satisfactorily calcined at about 150 C. to 300 C. lower hot zone temperature results in more satisfactory operation of the kilns and greater output.

While the plant has been illustrated as a schematic layout, it will be understood that the various elevators, conveyors, mixers and other devices are diagrammatically indicated and that other devices than those indicated may be used. In addition to pug mill 7, other mixing devices such as mullers and the like may be used to produce more thorough blending and mixing of the fine particle shale and water. Instead of feeding the coarse shale particles into kiln and the pelletized shale particles into kiln 15, each kiln may be fed with a mixed feed of coarse particles and pelletized particles.

While we have described our invention and the advantages thereof with particular reference to rotary calcining kilns, many of the advantages of our invention are also obtained where shaft or other forms of calcining kilns are used, and various other modifications and changes may be made from the procedure and plant layout here described Without departing from the spirit of our invention or the scope of the appended claims.

We claim:

l. The method of preparing an electric phosphate furnace feed from whole phosphate shale as mined and crushed to minus 3 inch size by increasing the clayey content of the fine fraction to permit pelletizing thereof, which comprises screening said shale to produce a fine particle size fraction containing the larger amount of the clayey material and a coarse particle size fraction substantially free from dust and containing a smaller amount of clayey material, calcining the coarse fraction to produce electric smelting furnace feed therefrom, moistening, pelletizing and drying the fine particle size fraction and passing the said pellets through a rotary kiln operated at a temperature above approximately 1050 C. to produce a pelletized electric smelting furnace feed therefrom.

2. The method of preparing an electric phosphate furnace feed from whole phosphate shale as mined and crushed to minus 3 inch size and containing about 18% of minus 200 mesh clayey binder material which comprises separating the whole crushed shale into a plus /2 inch and a minus /2 inch fraction and thereby producing a minus /2 inch fraction containing more than 25% of minus 200 mesh clayey binder material and a plus /2 inch fraction substantially free of dust, calcining the plus /2 inch fraction, moistening, pelletizing, drying and calcining the minus /2 inch fraction and using both fractions as a feed for electric phosphate furnaces.

3. The method of preparing an electric phosphate furnace feed from whole phosphate shale as mined and crushed to minus 3 inch size and containing about 18% of minus 200 mesh clayey binder material which comprises separating the whole shale into a plus inch and a minus A1 inch fraction and thereby producing a minus A inch fraction containing more than 25% of minus 200 mesh clayey binder material and a plus A inch fraction substantially free of dust, calcining the plus inch fraction, moistening, pelletizing, drying and calcining the minus /4 inch fraction and using both fractions as a feed for electric phosphate furnaces.

4. The method of preparing western phosphate shale for feed to electric smelting furnaces for the production of phosphorus and phosphatic products, which comprises crushing the mined shale into minus 3 inch particles separating the crushed mined shale into a coarse size fraction of about A to 3 inch particles and separating dust from said coarse size fraction, and a fine size fraction of about minus inch particles containing a larger percentage of minus 200 mesh clayey binder material than the percentage of said minus 200 mesh clayey binder material contained in the whole mined shale, moistening, pelletizing and drying the fine particle size fraction, passing the coarse size fraction substantially free of dust through a calcining kiln operated at hot zone temperatures of approximately 1050 to 1200 C., and passing the pelletized fine particle size fraction through a calcining kiln operated at a hot zone temperature of approximately 1050 to 1200 C. thereby producing from the whole of the mined shale a final product all of which may be used for feed to electric smelting furnaces.

References Cited in the file of this patent UNITED STATES PATENTS 2,125,263 Ahlmann Aug. 2, 1938 2,143,001 Curtis et al Jan. 10, 1939 2,167,120 Lloyd July 25, 1939 2,477,262 Mooser July 26, 1949 2,531,046 Hollingsworth Nov. 21, 1950 2,668,617 Houston Feb. 9, 1954 

1. THE METHOD OF PREPARING AN ELECTRIC PHOSPHATE FURNACE FEED FROM WHOLE PHOSPHATE SHALE AS MINED AND CRUSHED TO MINUS 3 INCH SIZE BY INCREASING THE CLAYEY CONTENT OF THE FINE FRACTION TO PERMIT PELIETIZING THEREOF, WHICH COMPRISES SCREENING SAID SHALE TO PRODUCE A FINE PARTICLE SIZE FRACTION CONTAINING THE LARGER AMOUNT OF THE CLAYEY MATERIAL AND A COARSE PARTICLE SIZE FRACTION SUBSTANTIALLY FREE FROM DUST AND CONTAINING A SMALLER AMOUNT OF CLAYEY MATERIAL, CALCINING THE COARSE FRACTION TO PRODUCE ELECTRIC SMELTING FURNACE FEED THEREFROM, MOISTENING, PELLETIZING AND DRYING THE FINE PARTICLE SIZE FRACTION AND PASSING THE SAID PELLETS THROUGH A ROTARY KILN OPERATED AT A TEMPERATURE ABOVE APPROXIMATELY 1050*C. TO PRODUCE A PELLETIZED ELECTRIC SMELTING FURNACE FEED THEREFROM. 